There are two major lines of lymphoid cells for distinct immunological functions: the thymus-derived T cells for cell-mediated immunity and the bursa-derived B cells for humoral immunity. Separation of these two cell lines can be easily accomplished in the chicken by in ovo bursectomy, followed by injection of cylophosphamide at hatching. The proposed research is designed to study the immunological functions of avian T cells, using bursectomized chickens. The objective of the proposed project is to identify macromolecules of the T-cell surface involved in recognizing antigen (T-cell receptors) and to study the molecular events in the interaction between T and B cells. For this objective, the following experiments will be performed: (1) isolation of antigen-specific suppressor T cells; (2) isolation and characterization of suppressor factors regulating the humoral immune response to thymus-dependent antigen; (3) demonstration of antigen-binding cells by autoradiography; (4) enrichment of antigen-binding cells and (5) isolation and characterization of T-cell receptors from the enriched antigen-binding cells.